1. Field of the Invention
The present invention relates to an oxidant (oxidizing agent) solution for synthesizing a heat-resistant electroconductive polymer and a method for preparing the electroconductive polymer, and more specifically to an oxidant solution for synthesizing, by a chemically oxidative polymerization, a heat-resistant electroconductive polymer which can stably hold its performance for a long term under a high temperature, and which be used in various electric or electronic devices utilizing an optical, electronic or electromagnetic characteristics of the electroconductive polymer, such as a battery cell, a capacitor, a diode, a display element, a secondary cell, a sensor, etc., and also a method for preparing the electroconductive polymer.
2. Description of Related Art
An electroconductive polymer can be said to be one which a conjugative bond highly develops in a polymer chain. A method for preparing the electroconductive polymer includes a chemical polymerization of polymerizing monomers by means of an oxidant or a catalyst, a method for heat-treating a chemical intermediate composed of a non-conjugative monomer, and an electrolytic polymerization for polymerizing the monomer by electrochemically oxidizing or reducing the monomer.
The method for preparing the electroconductive polymer, to which the present invention is directed, is chemical polymerization. In connection with chemical polymerization, there have been proposed a synthesis of a polyacetylene using the Ziegler-Natta catalyst, and a synthesis of a polyparaphenylene for polymerizing benzene by an oxidizing cationic polymerization.
The electroconductive polymer includes, in addition to the polyacetylene and the polyparaphenylene mentioned above, a fused ring electroconductive polymer such as polythiadile, polydiacetylenen and polyazulene, and a heterocyclic compound such as polypyrrole and polythiophene.
These electroconductive polymer are considered to be used as a solid electrolyte for an electrolytic capacitor and applied into a through hole in a printed circuit board since it is possible to easily polymerize into an electroconductive polymer having the conductivity of not less than 10 S/cm by a chemically oxidative polymerization using an oxidant such as an iron salt, for example, iron chloride (FeCl3).
Here, as one example of an industrial application of the electroconductive polymer, application of the polypyrrole into a tantalum solid electrolyte capacitor will be described with reference to FIGS. 4A and 4B. FIG. 4A is a sectional view of the tantalum solid electrolyte capacitor, and FIG. 4B is an enlarged sectional view of the portion "B" in FIG. 4A.
The shown tantalum solid electrolyte capacitor is formed as follows: A powder sintered body 4 of a metal tantalum which is a valve metal, is anodized in an electrolyte solution such as a phosphoric acid, so that the oxidized film 6 is formed on the tantalum body 4. The tantalum body 4 having the oxidized film 6 formed thereon is immersed in an aqueous solution containing the iron chloride (FeCl3), and then, immersed in a pyrrole monomer to form a polypyrrole. The oxidant solution held in the tantalum sintered body contacts with the pyrrole monomer, so that a polypyrrole layer 1 is formed on the oxidized film 6. In general, a desired polypyrrole thickness cannot be obtained with only one processing. Therefore, after repeatedly performing the above processing, a graphite layer 2 and a silver paste layer 8 are formed on the polypyrrole layer 1 in the named order.
In the iron chloride solution used as the oxidant for forming the polypyrrole, an iron (III) ion acts as the oxidant for the chemically oxidative polymerization for forming the polypyrrole, and a chlorine ion which is a pairing ion is introduced into the polymer as a dopant.
Since the conductivity of the polypyrrole is as large as 100 times that of manganese dioxide which was used in the prior art tantalum solid electrolyte capacitor, the tantalum solid electrolyte capacitor formed as mentioned above has an excellent high frequency characteristics. In addition, since the polypyrrole can be polymerized with only the contact of the pyrrole monomer with the oxidant solution, differently from the electrolytic polymerization, the polymerization is simple and therefore is high in industrial value.
As mentioned above, the chemical polymerization method for preparing the electroconductive polymer, using the iron salt such as the iron chloride, can form the electroconductive polymer having excellent conductivity by a simple process. However, in order to utilize the electroconductive polymer, it is required to have certain exposure properties, in particular, stability under a high temperature. In general, the electroconductive polymer is in a highly oxidized condition doped with anion, and therefore, is easily deteriorated with a coacting external factor such as heat, light, oxygen or water. This is a large hindrance in utilizing the electroconductive polymer as a device material. This is not exception in the polypyrrole having a relatively high thermal stability.
In order to increase the heat-resistant property of the polypyrrole, various methods have been proposed. For example, Japanese Patent Application Post-examination Publication No. JP-B-5-64970 and the corresponding U.S. Pat. No. 4,697,001 (the content of which are incorporated by reference in its entirety into this application) reports that a thermally stable polypyrrole can be obtained by oxidatively polymerizing the pyrrole, using a strong oxidant including cations of Fe.sup.3+, Cu.sup.2+, Ce.sup.4+ and (C6H5)3C.sup.3+, in the presence of a dopant anion selected from the group consisting of alkylsulfonate, arylsulfonate and fluorinated carboxylate.
In addition, Japanese Patent Application Pre-examination Publication No. JP-A-09-025417, (the content of which is incorporated by reference in its entirety into this application, and also an English abstract of JP-A-09-025417 is available from the Japanese Patent Office and the content of the English abstract of JP-A-09-025417 is also incorporated by reference in its entirety into this application) propose to use, as a dopant anion, a compound including at least one of --COOH-- and --OH--. This proposal is to prevent an oxidation deterioration of the polypyrrole starting from a proton dissociation of the pyrrole ring "N--H" or elimination of hydrogen of the .beta. site, by proton or hydrogen supplied from the dopant.
In fact, however, in the case of forming the electroconductive polymer such as polypyrrole by a chemical polymerization, even if the compound including at least one of --COOH-- and --OH-- is utilized as the dopant anion with the purpose of realizing the thermal stability of the electroconductive polymer, a satisfactory heat-resistant property cannot be obtained.
The reason for this is that: when the protonic solvent is used as a solvent for the iron salt, containing the dopant anion, the dopant anion cooperates with the iron (III) ion to form a chelate complex, which is taken or entrapped into the electroconductive polymer such as polypyrrole. The proton or hydrogen to be supplied from the dopant is not actually supplied, with the result that the thermal stability is lowered.